1. Field of the Invention
The present invention relates to machines for automatically unitizing a plurality of associated articles by wrapping with a stretchable plastic wrapping material.
2. Description of the Prior Art
Automatic stretch wrapping machines unitize a group of associated objects, such as individual boxes stacked on a pallet, by wrapping with a stretchable plastic wrapping material under tension. U.S. Pat. Nos. 4,077,179, 4,216,640, 4,235,062, 4,271,657, 4,300,326 and 4,302,920 disclose stretch wrapping machines which wrap a load with multiple layers of stretchable plastic wrapping material under tension.
The stretch wrapping machine disclosed in U.S. Pat. No. 4,077,179 has a pair of clamps which are mounted on a turntable which rotatably supports the load to be wrapped. Clamps grip the wrapping material for approximately 1 and 1/4 turns of the turntable before being withdrawn below the plane of the turntable. The wrapping material wraps against the outside of the clamps prior to their withdrawal which prevents maximum wrapping tension from being applied prior to clamp withdrawal, is wasteful of wrapping material and can result in a loose wrapping of the load when the clamp is retracted. The clamps grip a single thickness of the wrapping material across its width. Clamping of a stretch wrapping material by gripping a single thickness can cause tearing or severance of the film when high wrapping tension is used. Morever, withdrawal of film clamping clamps while the turntable is on the "fly" complicates turntable design. The wrapping material is severed by a combination clamp-cutting mechanism which severs the wrapping material across its full width when a pair of arms carrying the clamp-cutting mechanism are pivoted into vertical engagement with the wrapping material. The uniform vertical cutting of the wrapping material across the full width of the wrapping material is difficult to achieve. A non-uniform edge can make it difficult to attach the trailing edge of the severed wrapping material to the load. Vertical cutting mechanisms are also service prone and expensive. The severed wrapping material is brushed against the load by a brush to attach the trailing end of the wrapping material to the load.
U.S. Pat. No. 4,216,640 discloses an automatic stretch wrapping machine which clamps the leading edge of the stretch wrapping material with a pair of rotatable clamps which swing above the plane of the table to grasp the stretch wrapping material but pivot below the table during the initial wrapping of the load to eliminate the wrapping of the stretch wrapping material around the outside of the clamps. Thereafter, at the end of the wrapping cycle the stretch wrapping material is severed by the rotation of the clamps in the opposite direction to cause cutting surfaces which are mounted on the opposite side of the clamping arms to shear the stretch wrapping material. Thus, the clamping and shearing functions are performed by a single pair of rotatable arms which are initially rotated to grasp the stretch wrapping material and rotate it to a position below the surface of the turntable and thereafter after release of the leading edge of the wrapping material, the arms are rotated in such a direction that the cutting edges engage the stretch wrapping material at the end of the wrapping cycle to sever it.
U.S. Pat. No. 4,235,062 discloses a stretch wrapping machine utilizing wrapping material of reduced width to form a unitized load with breathing capability. The stretch wrapping material is reduced in width by a roping mechanism. The turntable clamps grab the leading edge of the wrapping material at a position above the turntable. The turntable clamping assembly is identical to that described in U.S. Pat. No. 4,077,179. The trailing end of the stretch wrapping material is tucked underneath a wrap of the stretch wrapping material which encircles the load and the outside of the turntable clamps. The turntable clamps are then pivoted to a position below the turntable surface which causes the wrap of stretch wrapping material which has been held away from the load to snap back toward the load and capture the end which has been tucked underneath it between the load surface and the clamps. Thereafter, the turntable clamps move upward and engage the new leading edge of the stretch wrapping material which is held in the clamps of the shearing mechanism.
U.S. Pat. No. 4,271,657 discloses an automatic stretch wrapping machine in which the leading and trailing edges of the stretch wrapping material are formed into ropes to facilitate their attachment together by a ring type of closure. The leading end of the stretchable wrapping material is secured by a clamping mechanism during the rotation of the turntable. A gathering arm is pivoted toward the clamp mechanism to overlay the trailing edge of the wrapping material, which has been formed into a rope, in proximity to the clamped leading edge of the wrapping material which is also in the form of a rope. The leading and trailing end sections of the wrapping material are tied together in a ring. Upon tying, the clamp is opened to release the leading edge of the wrapping material. Thereafter, the clamp is closed to hold the trailing rope section in a position upstream from a cutting mechanism which is disposed adjacent to the load. The cutting mechanism is activated to sever the trailing end of the rope section. The gathering arm is then rotated back to its original position and the wrapped load is transported away from the turntable. The clamped trailing end rope section becomes the leading end rope section of the next wrapping operation.
U.S. Pat. No. 4,300,326 discloses an automatic stretch wrapping machine which utilizes a clamping mechanism that is disposed below the surface of the turntable. The leading end of the wrapping material is grasped in the clamp mechanism. The trailing end of the wrapping material is formed into a rope-like configuration which is positioned adjacent the leading end by an actuator. The leading and trailing ends are tied together. The clamp mechanism then releases the leading end of the wrapping material and clamps the trailing end of the wrapping material at a point upstream from the point where the wrapping material is severed by the cutting mechanism.
U.S. Pat. No. 4,302,920 discloses an apparatus for wrapping a load with a stretchable plastic wrapping material. The patent discloses the prestretching of the wrapping material to produce an elongation which resists stresses during wrapping which could tear the film. It is preferred that the degree of elongation is at least 50% to avoid web breakage. A roping mechanism is disclosed.
U.S. Pat. No. 4,204,377 discloses a process for automatically spiral wrapping a unitary package with a netting material. The apparatus is similar to that disclosed in U.S. Pat. No. 4,235,062.